DC to DC (DC-DC) converters are in wide-spread use in portable electronic devices, which are primarily supplied with power from batteries. The portable electronic devices may be mobile terminals or portable entertainment equipment such as mobile phones, MP3 players, headphones, speakers, cellular phones, laptop computers etc.
Such portable devices often contain several sub-circuits with their own DC voltage requirements different from a DC supply voltage supplied by the battery or an external power supply. Furthermore, the DC supply voltage delivered by the battery declines as stored energy is drained.
Switch-mode DC-DC converters are operable to convert an incoming or input DC voltage level to another DC voltage level by temporarily storing the input energy and subsequently releasing stored energy to a DC output voltage node or terminal. The temporary energy storage may be in either magnetic components (for example inductors, transformers) and/or capacitive components.
By adjusting the Pulse Width Modulated (PWM) Duty Cycle of one or more controllable switches (the ratio of on/off time), the amount of power transferred can be controlled.
In a step-down or buck-mode converter a generated DC output voltage lies inside a DC voltage range between DC input voltage rails supplying power to the buck-mode converter. In a step-up, or boost-mode, DC-DC converter the DC output voltage lies outside the DC voltage range between the DC input voltage rails supplying power to the boost-mode converter. The DC output voltage may be negative relative to ground.
Both buck and boost mode converters comprise an inductor and two controllable switches coupled to the inductor. Each controllable switch is typically a controllable transistor or and a diode. The controllable switches control charging and discharging of the inductor. The DC-DC converter alternates between connecting the inductor to the DC input voltage, in order to store energy in the inductor, and discharging the inductor into a load.
Magnetic DC-DC converters are generally classified in one of two types according to a flow of current in its magnetic component (for example inductor or transformer):                Continuous mode where the level of current fluctuates in the magnetic component, but never goes down to zero;        Discontinuous mode where the current level fluctuates during a conversion cycle and drops to zero at an end of each conversion cycle.        
US2005/0088160 discloses a multi output DC-DC converter provided with a main switch which is turned on and off at a predetermined cycle, and which applies the input DC voltage of the DC power source to the inductor when turned on; a plurality of diodes which rectify a voltage generated in the inductor when the main switch is off; an auxiliary switch connected in series to each of the plurality of diodes; and a plurality of capacitors connected to the respective series circuits of the plurality of diodes and the auxiliary switch, which respectively output a plurality of output voltages.
US2007/0262760 discloses a single-inductor DC-DC convert capable of delivering multiple DC output voltages. One of the output voltages is always higher than the input voltage, while other output voltages may be higher or lower than the input voltage. The DC-DC converter requires no input power switch connected between the input voltage source and the power inductor. The DC-DC converter delivers power to all output voltages during the same switching cycle. The highest output voltage is used to reset the inductor current.
U.S. Pat. No. 6,075,295 discloses a DC-DC converter delivering two or more DC output voltages from one input voltage and one inductor. Each DC output voltage is formed by a switch controlling the charge delivered to each of the corresponding output capacitors. The controller utilizes pulse width modulation to deliver the correct amount of energy to each load and to synchronize the order of energy delivery to the loads. A predetermined DC voltage level for each load is changed by re-programming the controller thereby modifying the duration and frequency of the pulse received by the respective load from the charging inductor.
US2004/0135562 discloses a switch mode power supply for producing a plurality of DC output voltages using a single inductor. The inductor is provided with opposing first and second terminals. A first switch is connected between the first terminal and a source of electrical power to charge the inductor. A negative output of the power supply comprises a first capacitor and a second switch which switches electrical current from the first terminal to the negative output. A positive DC output of the switch mode power supply comprises a second capacitor and a third switch which switches electrical current from the second terminal to the positive output. Energy is stored in the inductor during an inductor charging cycle by closing the first switch. The stored energy is transferred to at least one of the negative and positive outputs during an inductor discharge cycle.